The present invention relates to modulation systems and more particularly to a feedback controlled modulation system for an amplifier having an input-output characteristic including a linear portion and at least a lower non-linear portion, and which uses an envelope detector to produce the feedback signal, as exemplified in FIG. 1.
The following description will be concerned with a Class C power amplifier. However, it is to be understood that the feedback system of the present invention is applicable to any type of amplifier having a non-linear input-output characteristic.
As is well understood, Nyquist's criterion must always be satisfied to guarantee feedback loop stability; and it may be helpful to note that this basic requirement can often be straightforwardly satisfied by having just a single narrow band stage in the feedback loop, with all other stages in the loop supplying a bandwidth greater than a certain critical value.
An important practical accomplishment of the present invention is that it allows the improvement to be described hereinbelow to be practically accomplished without reducing the above described required excess loop bandwidth of the system, thus allowing Nyquist's criterion to continue to be satisfied.
As illustrated in FIG. 3 of the drawing, the Class C power amplifier and the envelope detector, as employed in a Vortac transmitter have an input-output characteristic which includes an upper non-linear portion 1, a linear portion 2 and a lower non-linear portion 3. As illustrated in FIG. 3, there is appreciable incremental gain in the linear portion 2 with only very small varying incremental gain in the non-linear portions 1 and 3.
When employing solid state power devices for the Class C power amplifier the output waveform is also a bad function of "time on" so that the gain is also a function of the time width of the signal driving the amplifier.
The non-linear input-output characteristic illustrated in FIG. 3 is inherent in Class C amplifiers and in envelope detectors. Because of this non-linear input-output characteristic, prior art feedback systems, such as illustrated in FIG. 1, which will supply enough loop gain to produce desired system results in the linear portion 2 of the characteristic illustrated in FIG. 3, lose their loop gain in portions 1 and 3 of FIG. 3 and, therefore, unsatisfactory system performance results in these non-linear portions.
A simple procedure for attempting to overcome the above described limitations in portions 1 and 3 of FIG. 3 is to add a compensating shaped gain circuit in the feedback loop as illustrated in FIG. 2 to produce a gain characteristic which is a function of the amplitude of the modulating signal so that the effective input-output characteristic of the difference amplifier-power amplifier combination is linear throughout a desired dynamic range (i.e. with a Class C non-linear characteristic as part of a system, an overall compensated linear characteristic can never be obtained over an infinite dynamic range).
The above described simple procedure will not succeed for two practical reasons:
1. The added stages required to supply the large amount of additional shaped gain required usually unacceptably reduce the required excess loop bandwidth required for stable feedback performance. PA1 2. Most importantly from a practical point of view, the usual envelope detector cannot linearly detect the low outputs involved in portion 3 of FIG. 3, and in this region it, therefore, does not at all supply the correct feedback signal to the difference circuit. PA1 1. First, the gain of the reference amplifier which drives the difference circuit is shaped as a function of the modulating signal level in such a way that, with feedback removed, the resulting reference amplifier-power amplifier combination is approximately linear throughout the desired dynamic range of the system as illustrated in FIG. 4. Because the reference amplifier of FIG. 5 is not in the feedback loop, adding stages to this amplifier to supply the large amount of amplitude controlled increased gain required to compensate for the non-linear region 3 of FIG. 3 does not affect the loop gain excess bandwidth and, therefore, feedback stability is not destroyed by this procedure. PA1 2. Next, and most importantly, the linear range of the usual envelope detector is extended so that it covers the desired dynamic range of the system. This is accomplished by an arrangement fully disclosed in my co-pending application Ser. No. 075,613, filed Sept. 14, 1979. Here also it is important to note that when the disclosure of the above-cited co-pending application is used, the linear dynamic range of an envelope detecting system can be successfully extended in a practical way while still supplying enough excess bandwidth to satisfy the necessary feedback loop stability criteria. PA1 3. The gain of the extended range envelope detector system is then shaped so that it changes as a function of the modulating signal level in approximately the same manner that the reference amplifier gain is changed as described in 1 above. An important practical advantage of this extended range envelope detector is the fact that this gain shaping can be accomplished after detection; i.e. it does not have to be done in the auxiliary wide band limiting RF amplifier which is used in the extended range envelope detector system, and that it can be practically accomplished with sufficient excess bandwidth to preserve loop stability. PA1 4. Two control loops are provided for the exciter to ensure that the output signal of the exciter has an extremely flat top and the exactly correct amplitude to cause the power amplifier to operate in the extended linear range of its Class C characteristic. PA1 5. Taking all of the above together, the system of the present invention is capable of meeting or exceeding the above-indicated stringent spectrum specification.
In addition to the foregoing disadvantages of the systems of FIGS. 1 and 2, neither of these systems solve the problem of automatically ensuring that the input signal to the power amplifier has the exactly correct amplitude to cause the system to reach the threshold of its linearized Class C characteristic.
One of the major engineering problems encountered in attempting to meet operating specifications for a Class C transmitter, such as a Vortac transmitter, is to meet the spectrum specifications which requires providing a given spectrum to -50 to -60 db (decibels) down from the peak of spectrum. Neither of the systems of FIGS. 1 and 2 are capable of meeting this spectrum specification.
The system of the present invention illustrated in FIG. 5 overcomes all of the above-mentioned disadvantages by operating as follows:
When the above procedures are accomplished, it is possible to successfully provide a real time feedback controlled modulation system which overcomes the above-mentioned disadvantages of the prior arrangements caused by the low gain, non-linear, transfer characteristic portions 1 and 3 of FIG. 3 of Class C amplifiers and envelope detectors which are basic parts of many transmission systems.